Various material searches have been conducted with the aim at forming high-efficiency solar cells. It has been recently reported that high-efficiency solar cells can be formed using chalcopyrite semiconductors, such as CuInSe2 (CIS), which are compound semiconductors, and such cells attract attention as next-generation solar cells alternative to silicon solar cells. For example, Patent Literature 1 proposes a method for producing high-quality chalcopyrite nanoparticles. Although it is considered that devices for light-electricity conversion are easily produced by using chalcopyrite nanoparticles, there is the worry about future stable supply of In because rare element In is contained, and search for alternative materials is continued.
Materials having received attention quite recently include Cu2ZnSnS4 (CZTS) semiconductor. The CZTS semiconductor has a structure in which In in a CIS semiconductor is substituted by Zn and Sn and attracts attention as a material for high-efficiency solar cells. For example, in Non-Patent Literature 1, a solar cell using a CZTS semiconductor is formed as follows. First, a soda lime glass substrate is sputter-coated with molybdenum, and a CZTS thin film is formed thereon by a sulfurization method. Then, a buffer layer composed of CdS is deposited by a solution growth method, a window layer composed of ZnO:Al is deposited by a rf sputtering method, and a comb-shaped collector electrode composed of Al is deposited by a vacuum deposition method. The sulfurization method for forming the CZTS thin film is a two-stage forming method including forming a Cu—Zn—Sn—S precursor using a ternary simultaneous sputtering apparatus and then heat-treating the precursor in a hydrogen sulfide atmosphere after transferring it into an annealing chamber.